Ichthyosauria

Ichthyosauria (from the Ancient Greek ichthys, "fish," and sauros, "lizard," coined by Charles König in 1818 and Sir Richard Owen in 1840 after the pioneering Early Jurassic fossil discoveries of Mary Anning along the cliffs of Lyme Regis, Dorset, England in 1811–1812) is an extinct order of fully aquatic, pelagic diapsid marine reptiles (Ichthyopterygia, Sauropsida). Originating in the earliest Triassic (Olenekian, ~250–248 million years ago)—less than 3 to 4 million years after the end-Permian "Great Dying" mass extinction (251.9 Ma)—ichthyosaurs ruled the world's open oceans for 160 million years before going extinct at the Cenomanian–Turonian boundary (early Late Cretaceous, ~90–94 Ma), roughly 25 million years before the end-Cretaceous asteroid impact.

Representing the textbook paradigm of macroevolutionary convergent evolution in vertebrate zoology, derived Jurassic and Cretaceous parvipelvian ("fish-shaped") ichthyosaurs (Ichthyosaurus, Stenopterygius, Ophthalmosaurus, Platypterygius) independently evolved a streamlined, low-drag thunniform (tuna-, lamnid-shark-, and dolphin-like) hydrofoil body plan—complete with a pointed rostrum, a tall fibrous dorsal fin, rigid hydrofoil pectoral and pelvic paddles, and a vertical crescent-shaped lunate tail fluke—while Triassic giant shonisaurids (Shonisaurus sikanniensis and the 2024 Rhaetian giant Ichthyotitan severnensis) reached lengths of 21 to 26 meters and body masses of 50 to 80 metric tons, rivaling modern fin and sperm whales as the largest marine reptiles in Earth's history.

1. Transition from Anguilliform (Eel-Like) to Thunniform (Tuna-Like) Swimming: As traced by Ryosuke Motani (1999, 2005):
   • Earliest Triassic basal ichthyopterygians (Chaohusaurus, Utatsusaurus, and Grippia, ~248 Ma) had long, flexible bodies with 40+ presacral vertebrae, un-bent tails, and distinct wrist/ankle bones, swimming by whole-body lateral undulation (anguilliform swimming).
   • By the Late Triassic and Early Jurassic (Parvipelvia / Thunnosauria: Ichthyosaurus, Stenopterygius), the trunk was shortened and stiffened into a rigid spindle (reducing drag and lateral recoil), the pelvic girdle was reduced and detached completely from the vertebral column, and the posterior caudal vertebral column acquired a sharp downward bend (tailbend) that supported the lower lobe of a symmetrical, vertical crescent-shaped lunate caudal fin (while the upper lobe was supported by stiff collagenous fibers, exact opposite to the upturned heterocercal tail of sharks and horizontal tail flukes of cetaceans!).
2. Hyperphalangy and Hyperdactyly (Corn-on-the-Cob Flipper Mosaic): Inside the stiff, steering pectoral and pelvic paddles of derived ichthyosaurs, the radius, ulna, carpals, metacarpals, and phalanges lost their cylindrical shafts and flattened into a tight, inflexible pavement of polygonal bony discs wrapped in collagenous fibrous skin. To widen and lengthen the hydrofoil blade, ichthyosaurs evolved extreme hyperphalangy (multiplying the number of phalanges per finger from the ancestral 3–5 up to 20–30 phalanges per digit) and hyperdactyly (polydactyly)—adding supernumerary pre-axial and post-axial rows of fingers so that Caypullisaurus and Platypterygius possessed up to 8 to 10 parallel digits in a single flipper!
3. The Largest Eyes in Vertebrate History (Ophthalmosaurus and Temnodontosaurus): How did Jurassic and Cretaceous ichthyosaurs hunt elusive coleoid squids and belemnites in the dark mesopelagic twilight zone (500 to >1,000 meters depth) without ultrasonic echolocation (since CT scans show that ichthyosaurs lacked the osteological ear isolation required for whale-like biosonar)? They evolved the largest eyes ever grown by any animal on Earth (matched only by the modern colossal squid, Mesonychoteuthis):
   • In the 4-meter Jurassic ichthyosaur Ophthalmosaurus icenicus ("eye lizard"), the eye sockets fill almost the entire side of the skull (~22–23 cm diameter), and in the 9-meter Early Jurassic apex predator Temnodontosaurus, each eyeball measured 26 to >30 centimeters (1 foot) in diameter—the size of a dinner plate or basketball!
   • Each enormous eyeball was reinforced against water pressure distortion during rapid swimming by a circular ring of 14 to 17 overlapping bony plates inside the sclera (the sclerotic ring) and contained a gigantic spherical lens with an ultra-fast optical focal ratio (f-number ≈ 0.85, comparable to a night-vision camera lens!). As calculated by Ryosuke Motani, Rothschild, and Wahl (Nature, 1999), an Ophthalmosaurus eye had a photon-gathering sensitivity low enough to visually spot bioluminescent or silhouetted squid at >500–1,000 meters depth, while fossilized Ophthalmosaurus humeral heads exhibit avascular bone necrosis ("the bends" / decompression sickness) from diving rapidly into the deep sea!
4. Obligate Viviparity (Tail-First Live Underwater Birth) From a Terrestrial Ancestor: Because an adult ichthyosaur's pelvic girdle was disconnected from the spine and its flippers could not support its body weight on land, ichthyosaurs could never crawl onto beaches like sea turtles to lay hard-shelled eggs. Exquisitely preserved fossils from the Early Jurassic Posidonia Shale of Holzmaden, Germany (~182 Ma) show hundreds of gravid female Stenopterygius quadriscissus with 1 to 11 fully articulated embryos inside their body cavities—including specimens preserved in the very act of giving birth tail-first (just like modern dolphins and whales, so the newborn's head emerges last and it can swim immediately to the surface to take its first breath!). Remarkably, in 2014, Motani et al. discovered a maternal fossil of the earliest Triassic basal ichthyopterygian Chaohusaurus (~248 Ma) preserved giving birth head-first—indicating that viviparity in the ichthyosaur lineage actually evolved on land in their terrestrial diapsid ancestor before they entered the sea and switched to tail-first aquatic delivery!
5. Endothermy, Subcutaneous Blubber, and Dark Melanosome Countershading: In 2014 and 2018, Johan Lindgren, Maria E. McNamara, Nicholas R. Longrich, and colleagues analyzed a soft-tissue fossil of Stenopterygius (specimen MH 432) using pyrolysis-gas chromatography/mass spectrometry, synchrotron X-ray fluorescence, and immunohistochemistry. They discovered two remarkable features:
   • A thick layer of subcutaneous adipose blubber (previously known only in mammalian whales/seals and the leatherback sea turtle!), confirming along with bone phosphate δ¹⁸O thermometry (Bernard et al., 2010: body temperatures of 35–39 °C in 12–16 °C seawater) that ichthyosaurs were homeothermic endotherms; and
   • Smooth, completely scale-free skin pigmented with dendritic melanophores in a classic pelagic countershaded pattern (dark dorsal back and light ventral belly, or uniformly dark brown/black in deep-diving species like modern sperm whales!).

Why did ichthyosaurs go extinct 25 million years before the end-Cretaceous asteroid impact?

Whereas plesiosaurs and mosasaurs survived right up to the 66 Ma Chicxulub asteroid impact, the last surviving ichthyosaurs (Ophthalmosauridae such as Platypterygius) vanished abruptly at the Cenomanian–Turonian boundary (~93.9–90 Ma). As analyzed by Valentin Fischer and colleagues (Nature Communications, 2016), ichthyosaurs did not decline slowly throughout the Early Cretaceous (in fact, Albian ichthyosaurs were still taxonomically diverse), but suffered a two-phase extinction during the Cenomanian: first, an evolutionary bottleneck reduced their ecological disparity to slow-evolving apex/deep-diving forms, and second, the Cenomanian–Turonian Oceanic Anoxic Event (OAE2)—triggered by submarine volcanic large-igneous-province outgassing, extreme greenhouse warming, and global deep-water deoxygenation—wiped out their primary belemnite/ammonite prey and deep-water habitats, leaving their vacated ecological niches open to be colonized by radiating mosasaurs (giant marine squamate lizards) and pachycormid/lamniform fishes!

1. Motani, R., Rothschild, B. M., & Wahl, W., Jr. (1999). Large eyeballs in diving ichthyosaurs. Nature, 402(6763), 747–748. doi:10.1038/45435
2. Motani, R. (2005). Evolution of fish-shaped reptiles (Ichthyopterygia) in their physical environments and constraints. Annual Review of Earth and Planetary Sciences, 33, 395–420. doi:10.1146/annurev.earth.33.092203.122707
3. Motani, R., Jiang, D.-Y., Tintori, A., Rieppel, O., & Chen, G.-B. (2014). Terrestrial origin of viviparity in Mesozoic marine reptiles indicated by Early Triassic embryonic fossils. PLOS ONE, 9(2), e88640. doi:10.1371/journal.pone.0088640
4. Fischer, V., Bardet, N., Benson, R. B. J., Arkhangelsky, M. S., & Friedman, M. (2016). Extinction of fish-shaped marine reptiles associated with reduced evolutionary rates and global environmental volatility. Nature Communications, 7, 10825. doi:10.1038/ncomms10825
5. Lindgren, J., Sjövall, P., Thiel, V., Zheng, W., Ito, S., Wakamatsu, K., ... & Schweitzer, M. H. (2018). Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature, 564(7736), 359–365. doi:10.1038/s41586-018-0775-x
6. Lomax, D. R., de la Salle, P., Perillo, M., Reynolds, J., Reynolds, R., & Waldron, J. F. (2024). The last giants: New evidence for giant Late Triassic (Rhaetian) ichthyosaurs from the UK. PLOS ONE, 19(4), e0300289. doi:10.1371/journal.pone.0300289